The maximum reach of long-haul, high-speed transmission systems is limited by non-linear signal distortions. Non-linear signal distortions can typically be separated into two categories; namely inter-channel non-linear signal distortions and intra-channel non-linear signal distortions. For ultra-long transmission systems, e.g., 40 Gb/s, intra-channel effects within a single channel, as opposed to inter-channel effects between WDM channels, are the main sources of non-linear signal distortion.
Typically, the three major sources of intra-channel, non-linear signal distortions are four wave mixing, cross-phase modulation, and self phase modulation. Four wave mixing is a non-linear phenomenon involving generation of a new signal starting from the interaction between existing signals. The existing signals can be the various frequency components of an optical signal in a transmission system. Four wave mixing is especially pronounced in optical fibers having low dispersion within the signal band, i.e., within the range of signal wavelengths.
Cross-phase modulation occurs when signals induce phase distortion on co-propagating frequencies and group-velocity dispersion converts this phase distortion into amplitude distortion. Cross-phase modulation is especially troublesome in fibers having relatively high dispersion within the signal band.
Self-phase modulation is introduced or experienced when an optical signal of high power is launched into a standard fiber. The self-phase modulation effect appears as a pulse broadening and associated pulse distortion within individual wavelength channels. All of these non-linear signal distortions restrict the usable bandwidth of each optical carrier in a transmission system.